Almost every modern electronic product, ranging from toys to massive computers, now uses integrated circuits (“ICs”). ICs are generally made using photolithographic processes that involve manufacturing a template containing patterns of the electrical circuit as transparent and opaque areas. The patterned template is referred to as a “reticle” or “mask”. Generally, a number of these masks are required for manufacturing a complete device on the wafer.
A radiation source, such as a light, is used to copy or “pattern” multiple images of the mask onto a photosensitive material, such as a photoresist, on the surface of a silicon wafer. Once features are patterned on the photoresist, further processing is performed to form various structures on the silicon wafer. The completed wafer is then cut (or “diced”) to form the individual ICs.
Engineers typically use computer-aided design (“CAD”) to create a schematic design of the mask. One technique, Levenson phase shifting, also known as alternating aperture phase shifting, is used to create small features on ICs. Such small features are generated by a pair of areas in the mask called shifters.
One such mask is an alternating element phase shift mask (“PSM”) that normally includes a substantially transparent substrate composed, for example, of quartz. Phase-shifting material is situated in regions on the mask substrate to provide a phase shift to light radiation as it passes through the transparent areas of the mask. The phase-shifting regions can be formed in several ways, such as by depositing patterns of transparent films of appropriate thickness on the quartz substrate, or by etching vertical trench patterns into the quartz substrate.
The phase-shifting material may be, for example, silicon nitride or other suitable transparent materials such as oxides or oxynitrides. It may also simply be a thicker (or thinner) region of the same substrate material (e.g., quartz). In such an alternating element PSM, discrete non-phase-shifting components are then disposed alternately adjacent to discrete phase-shifting components.
An attenuated PSM is a PSM that contains discrete layers of absorbers, composed for example of chromium, disposed on the mask substrate. The absorber layers selectively attenuate the light that is passed therethrough.
Hybrid attenuated-unattenuated PSMs can also be combined with alternate element PSMs to provide more complex PSM designs.
Shifters are arranged to exploit the changes in the phases of the light that passes through them. For example, two shifters can be configured on a mask to shine light on the same region of a photoresist. In a region on the photoresist where the light passing through one of the shifters is in phase with the light passing through the other shifter, a feature can be created on the photoresist that is narrower than the distance between the two shifters. Where the light passing through the shifters is out of phase, no feature is created. By reducing the distance between the two shifters, very small features can be created on the photoresist. The width of the feature can be considerably less than could be produced by the same optical system without phase shifting.
In a trenched PSM, the trenches are arranged to alternate with untrenched areas in order to provide alternating regions of phase-reversed light and unreversed light. However, the stray light from the sidewalls of the trenches usually interferes with the normal (desirable) incident light. This interference lowers the intensity of the light that exits from the trenches. Due to the lower intensity of the light that exits from the trenched areas, the alternating trenched and untrenched areas will not provide equal light intensities on the target photoresist. The resulting intensity imbalance then causes the printed photoresist features on the wafer to have placement errors.
Opaque areas, typically of chrome, are usually provided on PSM masks to block the light in areas where features are not to be formed. One known solution for the intensity imbalance issue is to provide undercuts beneath the chrome and to bias (i.e., to thin) the trench chrome opening. However, undercuts limit the minimum chrome size due to problems with peeling of the chrome. Biasing reduces the chrome that remains and hence also contributes to the chrome peeling issue.
Another mask solution is a sidewall chrome alternating aperture mask (“SCAAM”) in which the sidewalls of the trenches are coated with a light-absorbing layer of chrome. The SCAAM technique requires less biasing and undercutting due to the chrome that is deposited on the sidewall for eliminating the problem of stray sidewall light.
Unfortunately, however, there are issues with the SCAAM technique that limit its use. One issue is the complexity of forming the additional sidewall chrome layer. This requires an additional chrome layer deposition process, which includes an additional mask-making process for the chrome deposition, additional photoresist processing that includes a complicated resist topography, etching of the chrome, and so forth. These issues limit the application of the SCAAM technique.
Thus, a need still remains for a trenched PSM method and apparatus that wilt effectively yet inexpensively eliminate intensity imbalances between the alternating trenched and untrenched areas of the mask. In view of the ever-increasing need to save costs and improve efficiencies, it is more and more critical that answers be found to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.